Hub-bearing units of the flanged type, for applications to drive wheels of motor vehicles, are known from the prior art. The document EP 2 602 123 A1, for example, describes a hub-bearing unit, which is asymmetrical in this case, for the wheel of a motor vehicle, comprising a flanged hub rotatable about an axis of rotation, a flange fixed to the flanged hub and placed transversely to the axis of rotation, a fixed ring positioned radially outside the flanged hub and provided with races axially spaced apart from each other, and two crowns of rolling bodies (such as balls) positioned between the fixed ring and the flanged hub. The flanged hub integrally forms a radially inner race for the axially outer crown of balls, while the radially inner race for the crown of axially inner balls is formed on an inner ring of the bearing, fitted radially and externally on to the flanged hub.
An embodiment of this type, especially when used in applications which are demanding in terms of transmitted loads, creates considerable local loads between the rings and the rolling bodies of the bearing; moreover, this embodiment cannot be used to produce a very strong or highly durable bearing.
In order to improve the performance, and especially the strength, of the bearing, the distance between the pressure centers must be increased. This may be done by increasing the diameter of the circumference of the centers of the rolling bodies (known as the pitch diameter) of the bearing. Such solutions are known and have been developed in order to improve performance to a substantial degree. Increasing the pitch diameter has the drawback that the volume, and therefore the weight, also increases dramatically, with the square of the value of the pitch diameter. This weight increase is usually unacceptable to motor vehicle manufacturers.
Another improvement may be made by providing a further increase in the diameter of the circumference of the centers of the rolling bodies, so that the constant velocity joint can be fitted into the bearing and the part known as the bell of the joint can be integrated with the hub, that is to say with the inner ring of the bearing. Clearly, the integration of the components enables both the weight and the cost of the whole unit to be reduced. The weight and cost can be reduced further by also integrating the small inner ring of the bearing, in the axially inner position, with the bell of the joint. In other words, the hub also acts as a single inner ring of the bearing and as the bell of the constant velocity joint.
Finally, in order to improve the integration of the unit further, the races of the rolling bodies are formed not only in a single fixed outer ring but also in a single rotatable inner ring. However, this configuration requires the development of a new configuration, particularly as regards the radially outer ring of the bearing, for mounting the hub-bearing unit on the knuckle of the suspension of the motor vehicle in such a way as to create a pre-loading condition.
If this were not done, the forces transmitted to the bearing during the operation of the vehicle would tend to detach it from the knuckle. A case in point is that of a left-hand front wheel that is turning to the right. The centripetal forces generated tend to pull the bearing out of the knuckle, and in this example they are directed from the axially outer (“outboard”) side to the axially inner (“inboard”) side.
The mounting configuration must also provide for the positioning of a speed sensor, for example the speed sensor controlled by the anti-lock braking system (ABS) of the wheel, which is usually placed on the axially inner side of the bearing.
A known solution for the stable mounting of the outer race of the bearing on the knuckle of the suspension provides for the bending of one edge of the outer ring by cold plastic deformation. The method normally used is that of orbital forming, carried out with suitable orbital machines. The bent edge of the outer race acts as a stop element for stopping the bearing with respect to the knuckle. The bent edge must be the edge of the axially outer side of the ring, to ensure the correct positioning of the ABS sensor on the axially inner side.
Another known solution is that in which the outer ring of the bearing is fixed to the knuckle by radial interference and by using a stop ring of the Seeger type, whose seat, formed on the knuckle, is located on the axially outer side. In this case, as in the previous one, the positioning of the stop ring on the axially outer side is due to the need to ensure the correct positioning of the ABS sensor.
Both of the solutions illustrated above therefore have a drawback associated with the fact that the weakest portion of the structure, namely the bent edge of the outer ring, or the housing seat and the Seeger ring itself, is located in the most stressed section, that is to say the axially outer side. Consequently, this section must have a considerable thickness, in order to withstand the forces acting on it. Clearly, therefore, more material must be used and the operating pressure of the orbital forming machine must be greater, resulting in a more difficult and costly assembly process.